In an optical disk, a Blu-ray Disc™ (BD) using a blue-violet semiconductor laser and a high-NA objective lens having a numerical aperture NA of 0.85 has been commercialized and has reached nearly a limit of optical resolution of an optical system. Thus, it is thought that increasing the number of recording layers will be dominant so as to achieve a larger amount of capacity. In recent years, a Blu-ray Disc having two recording layers or three recording layers is offered commercially and is used as a storage medium of a video recorder and a personal computer.
In this multilayer optical disk, detected lights from respective recording layers need to be nearly equal to each other in amount, so that as the number of layers is increased, the reflectivity of each of the recording layers is forced to be small. Hence, this presents a problem that a Signal to Noise ratio (SNR) of a readout signal from each of the recording layers is reduced.
Hence, a technique of amplifying a readout signal has been developed. As an example of the technique, the following technique is described in Japanese Unexamined Patent Publication No. 2008-310942. A weak light (signal light) reflected by an optical disk is mixed with light (reference light) that is split before being applied to the optical disk and is not applied to the optical disk and is amplified by interference. At this time, the mixed light is split into two components by a non-polarizing beam splitter (NPBS) and the components are transmitted through a half wave plate or a quarter wave plate. Then, a differential detection between light transmitted through a polarizing beam splitter (PBS) and light reflected by the polarizing beam splitter is performed, whereby four interfering lights of two split lights which are different from each other in a phase relationship are detected. When the optical disk is rotated, the optical disk fluctuates, which varies a light path length of the signal light, whereby the outputs of four interfering lights are varied. However, by selectively calculating four outputs, a readout signal can be steadily amplified.